Per this thread on FlyerTalk, many Priority Club Platinum members have had success registering the promo code 6186. I signed up for this two weeks ago, and my 5,000 point bonus posted today. It seems like it's not connected to anything other than being Platinum, but keep in mind that unless you were targeted, you're technically not entitled to this, so don't call Priority Club if this offer doesn't post to your account.
A big PLAT thanks for the tip! I just tried it, and it worked for me.